1. Field of the Invention
The present invention relates to the field of data storage. More particularly, the present invention relates to a method and a device for write-precompensating transitions written to a magnetic storage medium at high data rates.
2. Description of the Related Art
FIG. 1 shows an exemplary uncompensated non-return-to-zero (NRZ) write-magnetization pattern 10. FIG. 1 shows two exemplary transitions 11 and 12 that are each more than one write-clock period 16 from another transition, and are each referred to as an isolated transition. Two closest-spaced transitions, preceded and followed by one or more write docks without a transition, are referred to as a dibit. FIG. 1 shows two exemplary dibits 13 and 14. Similarly, three closest-spaced transitions are referred to as a tribit, four closest-spaced transitions form a tetrabit, and so on. FIG. 1 also shows a group of transitions 15 that is generally referred to as a burst. A dibit is a two-transition burst, a tribit is a three-transition burst, etc. The transitions in a burst of length n will be assumed to be numbered 1 through n, sequentially.
Several different detection schemes can be used for reading back magnetically recorded data, such as a PRML (Partial Response Maximum-Likelihood) scheme. For PR readback schemes, a single isolated transition (11, 12 in FIG. 1) on readback gives rise to a detected signal that is more that one clock period wide. For PR readback, the readback signal is shaped by filtering and equalization so that when the readback signal is sampled at the readback clock rate, which is the same rate as the write clock rate, an isolated transition ideally gives rise to two equal nonzero sample values that are separated by one read-clock period and with all other samples being zero, e.g., .+-.( . . . 0 0 1 1 0 0 . . . ).
The readback signals from adjacently-written transitions purposely overlap so that the two successive transitions of a dibit should ideally have a readback pattern of .+-.( . . . 0 0 1 0-1 0 0 . . . ). This readback pattern is the sum of the ideal readback patterns for two isolated transitions in which the second isolated transition is delayed in time by one clock period and its polarity is opposite that of the preceding transition. For PR, a burst of n transitions should ideally have a readback pattern of . . . 0 1 0.+-.1 0 . . . 0.+-.1 0 0 . . . , with n-1 zero samples between the two nonzero samples. The signs of the two nonzero samples are opposite when n is even. A dibit is thus a special case of a short burst having only one zero sample between the two nonzero samples.
Two other common detection schemes used for reading back magnetically recorded data are EPR (Extended PR) and E.sup.2 PR Extended Extended PR). For EPR, an isolated transition gives rise to an ideal equalized readback signal with three successive nonzero samples, .+-.( . . . 0 0 1 2 1 0 0 . . . ). For EPR, a burst of n transitions ideally produces a readback pattern having two equal nonzero samples .+-.(1 1), followed by n-2 zero samples, followed in turn by two equal nonzero samples .+-.(1 1). An isolated transition for E.sup.2 PR gives rise to four nonzero samples, .+-.( . . . 0 0 1 3 3 1 0 0 . . . ), for an ideal equalized readback pattern. For E.sup.2 R, a burst of n transitions should ideally produce three nonzero samples .+-.(1 2 1), followed by n-3 zero samples, followed by three nonzero samples .+-.(1 2 1). For E.sup.2 R, a dibit is a special case, producing the pattern .+-.( . . . 0 0 1 2 0-2-1 0 0 . . . ). For all these detection methods, the sign of the trailing nonzero samples of a burst is opposite that of the leading nonzero samples if n is even.
Write precompensation, that is, the shifting of write pulses in time with respect to the write clock, has long been used in magnetic recording to improve recording performance. For example, in both peak-detection and PR channels, precompensation is used to compensate for intersymbol interference (ISI) and nonlinear transitions shift (NLTS) effects. An NLTS effect occurs when the demagnetization fields from just-written transitions combine with the magnetic field of the write head to cause a transition being written to be shifted from its intended location on the magnetic medium. Write precompensation counteracts the NLTS so the transition is written at its intended location.
In conventional write precompensation for recording high-density transitions, the amount of precompensation used for a transition is determined by the distance of the transition from prior transitions. That is, conventional precompensation techniques compensate for head field interactions with fields from the pattern of magnetic transitions on the disk. Only transitions a short distance from a transition being written typically affect the amount of write precompensation needed because the strength of the interaction (with the demagnetization field from the previously written transitions) decreases very rapidly with distance. The amount of NLTS precompensation needed depends only on the physical distance(s) to the preceding transition(s) on the disk, with precompensation values being a constant fraction of the write clock for a given density of transitions on the disk. In this regard, conventional precompensation techniques are used to set the delay(s) of the second and subsequent transitions of a burst with respect to the write clock. At today's recording densities, NLTS precompensation is typically applied to a transition being written only if another transition was written on the immediately preceding write clock (first-order NLTS precompensation). For this reason, only the second and subsequent transitions of bursts in a data pattern wold have NLTS precompensation applied and the precompensation delay is the same for al such transitions. The amount of this precompensation is termed .epsilon..sub.1. At higher densities, a transition being written would be affected by the demagnetization field from transitions written up to two write clocks earlier. The magnitude of the transition shift from a transition occurring two clocks earlier is termed .epsilon..sub.2 and is much smaller than .epsilon..sub.1 because the distance is doubled. If a transition being written is preceded only by a transition written two clocks earlier, with no transition written one clock earlier, the transition shift is .epsilon..sub.2. If a transition being written is preceded by transitions written on both preceding write clocks, the transition shift is .epsilon..sub.1 -.epsilon..sub.2, with the minus sign being necessary because the transition written two clocks earlier is of the same polarity as the transition being written. Second-order write precompensation is used to adjust for NLTSs due to transitions written up to two write clocks earlier.
For magnetic recording at high data rates, difficulties arise when writing closest-spaced transitions, that is, transitions separated by only one write-clock tick or period. As the data rate increases, shifts in transitions occur that are caused only by high-data-rate writing effects. High-data-rate writing, as used herein, is defined to be the recording or writing of transitions on a disk when the period of the write clock used for recording is comparable to or shorter than time-dependent phenomena occurring in the write path, that is, the write channel, the arm electronics, the head, the disk, and all associated wiring. In other words, high-data-rate writing is the process in which the time needed for reaching writing conditions, whether caused by the head, the disk and/or the driving electronics, is comparable to or greater than the period of the write clock.
When a burst of transitions is written by high-data-rate writing, that is, a series of two or more minimum-spaced transitions is written, the head lacks sufficient time to reach a steady state during a single write-clock period. Thus, the state of the head at the start of each transition in a burst (after the first transition) is different than for isolated transitions (i.e., transitions that are not part of a burst). As a consequence of the potentially different initial states of the head for a burst of transitions, the time needed for reaching writing conditions differs for each transition in the burst, thus leading to improper transition placement.
A method is needed for write precompensating transitions for high-data-rate recording to allow higher data rates and recording densities. It must be emphasized that the density-dependent transition shifts described earlier do not disappear. The high data rate transition shits occur in addition to any density-dependent shifts and, in many cases, can exceed them in magnitude, making their treatment of secondary importance.